Electronic nudges to increase influenza vaccination uptake in young and middle-aged patients with heart failure: a prespecified analysis of the NUDGE-FLU-CHRONIC trial

Background Influenza vaccination rates remain suboptimal among patients with heart failure (HF) despite strong recommendations and associations with reduced HF-related morbidity and mortality. During the 2022-2023 influenza season, NUDGE-FLU demonstrated effectiveness of behaviorally informed electronic letter-based nudges in increasing influenza vaccination among older adults; however, it is currently unknown whether these interventions are effective in young and middle-aged individuals, where an even greater implementation gap exists. In this prespecified analysis of the ongoing NUDGE-FLU-CHRONIC trial, we explored the effects of behavioral nudges to increase influenza vaccination according to the presence or absence of HF at baseline and across HF subpopulations. Methods NUDGE-FLU-CHRONIC is a nationwide randomized implementation trial conducted during the ongoing 2023-2024 influenza season. The trial enrolled all Danish citizens aged 18-64 years who qualified for free-of-charge influenza vaccination due to preexisting chronic diseases. Participants were randomized in a 2.45:1:1:1:1:1:1 ratio to usual care or 1 of 6 different behaviorally informed letter-based nudging strategies delivered via the governmental Danish electronic letter system. The interventions were delivered prior to the influenza vaccination period on September 24, 2023, and the primary endpoint was receipt of influenza vaccination on or before January 1, 2024. Results The trial enrolled 299,881 participants, of whom 11,727 had HF at baseline. In the HF subgroup, median age was 57.6 years, 73.6% were male, and the median number of GDMT classes used at baseline was 3 (IQR 2-4). During the 2023-2024 season, influenza vaccination was obtained by 46.9% of participants with HF compared with 35.7% of those without HF. Among HF patients, influenza vaccination uptake was significantly higher in those with higher levels of baseline GDMT use (≥3 classes: 50.1% vs. ≤2 classes: 43.0%; p<0.001). Receiving any letter-based nudge resulted in increased influenza vaccination rates compared with usual care among participants with and without HF (HF: +13.5 percentage points [99.29% CI 10.8-16.1], p<0.001; no HF: +11.6 percentage points [99.29% CI 11.1-12.1], p<0.001; pinteraction=0.064) (Figure 1). Similar effectiveness was observed among patients with HF regardless of baseline use of SGLT2i or ARNi, the number of GDMT classes used at baseline, or whether patients had an ICD or CRT device. Each individual letter was effective in increasing influenza vaccination among participants with and without HF (Figure 2). Conclusions Less than half of the young and middle-aged patients with HF enrolled in NUDGE-FLU-CHRONIC received influenza vaccination during the 2023-2024 season in Denmark. Behaviorally informed electronic letter-based nudges were highly effective in increasing vaccination rates in participants with and without HF at baseline.